We present a new instrument, "Boreas", a cryogen-free methane (CH 4 ) preconcentration system coupled to a dual-laser spectrometer for making simultaneous measurements of δ 13 C(CH 4 ) and δ 2 H(CH 4 ) in ambient air. Excluding isotope ratio scale uncertainty, we estimate a typical standard measurement uncertainty for an ambient air sample of 0.07‰ for δ 13 C(CH 4 ) and 0.9‰ for δ 2 H(CH 4 ), which are the lowest reported for a laser spectroscopy-based system and comparable to isotope ratio mass spectrometry. We trap CH 4 (∼1.9 μmol mol −1 ) from ∼5 L of air onto the front end of a packed column, subsequently separating CH 4 from interferences using a controlled temperature ramp with nitrogen (N 2 ) as the carrier gas, before eluting CH 4 at ∼550 μmol mol −1 . This processed sample is then delivered to an infrared laser spectrometer for measuring the amount fractions of 12 CH 4 , 13 CH 4 , and 12 CH 3 D isotopologues. We calibrate the instrument using a set of gravimetrically prepared amount fraction primary reference materials directly into the laser spectrometer that span a range of 500−626 μmol mol −1 (CH 4 in N 2 ) made from a single pure CH 4 source that has been isotopically characterized for δ 13 C(CH 4 ) by IRMS. Under the principle of identical treatment, a compressed ambient air sample is used as a working standard and measured between air samples, from which a final calibrated isotope ratio is calculated. Finally, we make automated measurements of both δ 13 C(CH 4 ) and δ 2 H(CH 4 ) in over 200 ambient air samples and demonstrate the application of Boreas for deployment to atmospheric monitoring sites.
Abstract. The precise measurement of the amount fraction of atmospheric nitrous oxide (N2O) is required to understand global emission trends. Analysis of the site-specific stable isotopic composition of N2O provides a means to differentiate emission sources. The availability of accurate reference materials of known N2O amount fractions and isotopic composition is critical for achieving these goals. We present the development of nitrous oxide gas reference materials for underpinning measurements of atmospheric composition and isotope ratio. Uncertainties target the World Metrological Organisation Global Atmosphere Watch (WMO-GAW) compatibility goal of 0.1 nmol mol−1 and extended compatibility goal of 0.3 nmol mol−1, for atmospheric N2O measurements in an amount fraction range of 325–335 nmol mol−1. We also demonstrate the stability of amount fraction and isotope ratio of these reference materials and present a characterisation study of the cavity ring-down spectrometer used for analysis of the reference materials.
<p>Measurements of the four most abundant stable isotopocules of N<sub>2</sub>O (<sup>14</sup>N<sup>14</sup>N<sup>16</sup>O, <sup>15</sup>N<sup>14</sup>N<sup>16</sup>O, <sup>14</sup>N<sup>15</sup>N<sup>16</sup>O, and <sup>14</sup>N<sup>14</sup>N<sup>18</sup>O) can provide a valuable constraint on source attribution of atmospheric N<sub>2</sub>O. N<sub>2</sub>O isotopocules at natural abundance levels can be analyzed by isotope-ratio mass-spectrometry (IRMS) [1] and more recently optical isotope ratio spectroscopy (OIRS) [2]. OIRS instruments can analyze the N<sub>2</sub>O isotopic composition in gaseous mixtures in a continuous-flow mode, providing real-time data with minimal or no sample pretreatment, which is highly attractive to better resolve the temporal complexity of N<sub>2</sub>O production and consumption processes. Most importantly, OIRS laser spectroscopy is selective for position-specific <sup>15</sup>N substitution due to the existence of characteristic rotational-vibrational spectra.</p><p>By allowing both in-situ application and measurements in high temporal resolution, laser spectroscopy has established a new quality of data for research on N<sub>2</sub>O in particular and N cycling in general. However, applications remain challenging and are still scarce as a metrological characterization of OIRS analyzers, reporting factors limiting their performance is still missing. In addition, only since recently two pure N<sub>2</sub>O isotopocule reference materials have been made available through the United States Geological Survey (USGS), which however, only offer a small range of &#948;<sup>15</sup>N and &#948;<sup>18</sup>O values (< 1 &#8240;) and are therefore not suited for a two-point calibration approach [3].</p><p>This presentation will highlight the recent progress achieved within the framework of the EMPIR project &#8220;Metrology for Stable Isotope Reference Standards (SIRS)&#8221;, namely:</p><ul><li>(1) The development of pure and diluted N<sub>2</sub>O reference materials (RMs), covering the range of isotope values required by the scientific community. These gaseous standards are available as pure N<sub>2</sub>O or N<sub>2</sub>O diluted in whole air. N<sub>2</sub>O RMs were analyzed by an international group of laboratories for &#948;<sup>15</sup>N, &#948;<sup>18</sup>O (MPI-BGC, Tokyo Institute of Technology, UEA), &#948;<sup>15</sup>N<sup>&#945;</sup>, &#948;<sup>15</sup>N<sup>&#223;</sup> (Empa, Tokyo Institute of Technology) and &#948;<sup>17</sup>O (UEA) traceable to the existing isotope ratio scales.</li> <li>(2) The metrological characterization of the three most common commercial N<sub>2</sub>O isotope OIRS analyzers (with/without precon QCLAS, OA-ICOS and CRDS) for gas matrix effects, spectral interferences of enhanced trace gas concentrations (CO<sub>2</sub>, CH<sub>4</sub>, CO, H<sub>2</sub>O), short-term and long-term repeatability, drift and dependence of isotope deltas on N<sub>2</sub>O concentrations [4].</li> </ul><p>In summary, the authors suggest to include appropriate RMs following the identical treatment (IT) principle during every OIRS measurement to retrieve compatible and accurate results. Remaining differences between sample and reference gas composition have to be corrected, by applying analyzer-specific correction algorithms.</p><p>&#160;</p><p>[1] Toyoda, S. and N. Yoshida (1999). "Determination of nitrogen isotopomers of nitrous oxide on a modified isotope ratio mass spectrometer." Anal. Chem. 71(20): 4711-4718.</p><p>[2] Brewer, P. J. et al. (2019). "Advances in reference materials and measurement techniques for greenhouse gas atmospheric observations." Metrologia 56(3).</p><p>[3] Ostrom, N. E. et al. (2018). "Preliminary assessment of stable nitrogen and oxygen isotopic composition of USGS51 and USGS52 nitrous oxide reference gases and perspectives on calibration needs." Rapid Commun. Mass Spectrom. 32(15): 1207-1214.</p><p>[4] Harris, S. J., J. Liisberg et al. (2019). "N<sub>2</sub>O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison." Atmos. Meas. Tech. Discuss. (in review).</p><p>&#160;</p>
<p>This paper will describe the characteristics and performance of a system to prepare up to ten 50&#160;mL samples of pure CO<sub>2</sub> with on-demand <sup>13</sup>C/<sup>12</sup>C ratios, together with an optimized calibration system for measurements by Isotope Ratio Infrared Spectroscopy (IRIS) that has allowed measurement of &#948;<sup>13</sup>C and &#948;<sup>18</sup>O values with 0.02 &#8240; reproducibility (1 &#963;).</p><p>The needs for improved quality infrastructure and appropriate reference gases for CO<sub>2</sub> isotope ratio measurements has been a driver for recent research and development activities within the National Metrology Institutes, and the decision of the Gas Analysis Working Group of the CCQM to plan an international comparison (CCQM-P204) of capabilities of measurements of these quantities. The comparison will be coordinated by the BIPM, which has the mission of preparing the comparison samples, and the IAEA, who will assign their isotopic composition on reference scales. The BIPM has developed a preparation facility based on blending of different pure CO<sub>2</sub> sources of very different isotopic compositions, followed by cryogenic trapping and transfer to ten 50 mL cylinders. The target isotopic ratio <sup>13</sup>C/<sup>12</sup>C can be adjusted by accurate flow measurements.</p><p>A Carousel sampling system with bracketing reference gas calibration and dilution system has been designed at the BIPM to allow rapid and accurate analysis of prepared gas mixtures by IRIS. A key feature of the calibration system is to maintain identical treatment of sample and reference gases allowing two-point calibration of up to 14 samples, and appropriate flushing protocols to remove any biases from memory effects of previously sampled gases. Measurements are performed by the IRIS analyzer at a mole fraction of nominally 700 &#956;mol/mol CO<sub>2</sub> in air, by dilution of pure CO<sub>2</sub> gas controlled by individual low-flow mass flow controllers (0.07 ml/min), and with a feedback loop to control mole fractions to ensure that differences between references and sample gas mole faction stay below 2 &#956;mol/mol. This level of control is necessary to prevent biases in measured isotope ratios, the magnitude of which has also been studied with a sensitivity study that is also reported.</p><p>The Carousel and IRIS measurements have been validated using pure CO<sub>2</sub> samples prepared with the gas blending facility, covering a range in delta values of -1 &#8240; to -45 &#8240; vs VPDB, and in all cases measurement reproducibility over several days of testing of 0.02&#8240; or better (1 &#963;) were achieved for both &#948;<sup>13</sup>C and &#948;<sup>18</sup>O, with negligible memory effects.</p><p>Samples produced and characterized with the facility will be distributed to institutes participting in the CCQM-P204 comparison exrecise, with measurements foreseen in the first quarter of 2020.</p>
<p>We demonstrate the possibilities for continuous high precision in situ measurements of &#948;<sup>13</sup>C(CH<sub>4</sub>) and &#948;<sup>2</sup>H(CH<sub>4</sub>) for understanding regional CH<sub>4</sub> emissions and explain how advances in nascent measurement techniques looking at &#8216;clumped&#8217; CH<sub>4</sub> might improve our understanding on the global scale.</p><p>&#8216;Boreas&#8217; is a new fully automated sample-preparation coupled dual laser spectrometer system developed at the National Physical Laboratory, able to make accurate and precise simultaneous measurements of &#948;<sup>13</sup>C(CH<sub>4</sub>) and &#948;<sup>2</sup>H(CH<sub>4</sub>) through the measurement of isotopologue ratios of CH<sub>4</sub>. Average daily repeatabilities of <0.08 &#8240; for &#948;<sup>13</sup>C (n=10, 1 SD) &#160;and <1&#8240; &#948;<sup>2</sup>H of a compressed &#8216;background&#8217; air sample (1.9 ppm dry air amount fraction CH<sub>4</sub>) are achieved, making the measurements comparable to bulk isotope ratio mass spectrometry. These measurements are interspersed with air sample measurements from the roof of our building in west London, and we show the possibility to differentiate potential sources of CH<sub>4</sub> under different meteorological conditions.</p><p>We use a particle dispersion model (the Met Office&#8217;s NAME) and inverse method to predict the possible impact of the new continuous isotope ratios measurements on quantification of emissions from individual source sectors, should the technique be deployed to a tall tower network of monitoring sites in the UK.</p><p>Finally, our theoretical analysis is extended beyond the most abundant isotopologues of CH<sub>4</sub> to look at how analysis of the clumped isotopes might be able to impact our understanding of interannual variability in the global CH<sub>4</sub> burden. We incorporate measurements from emission sources and information on reaction rates into a global box model (with an inverse method) to show the added value of strategic &#8710;CH<sub>2</sub>D<sub>2</sub> and &#8710;<sup>13</sup>CH<sub>3</sub>D ambient air measurements relative to bulk isotope ratios alone.</p>
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